October 3 (HealthDay News) -- The standard test for checking the vision of young children cannot be completely trusted, Johns Hopkins University researchers report.

The test, called fixation preference test (FPT), is used to evaluate visual acuity, which is the ability of the eyes to focus images on the macula, the part of the eye that handles detailed vision.

"Current methods of vision testing in children 6 months to 30 months of age is likely not completely reliable," said lead researcher Dr. David S. Friedman, an associate professor of ophthalmology and international health at Johns Hopkins Wilmer Eye Institute. "Current methods of testing vision in very young children need to be re-assessed."

The report was published in the October issue of Ophthalmology.

For the study, Friedman's team evaluated visual acuity in 1,504 children aged 30 to 71 months. The researchers used a series of tests including FPT and the Amblyopia Treatment Study test (ATS).

For the FPT, each eye is covered and then uncovered, which determines how well the eyes maintain fixation. The test is designed to measure visual acuity in the best eye. The purpose of the ATS test is to measure visual acuity in children aged 30 months and older.

The researchers found that the FPT, which is considered the standard for testing vision in young children and the only test available to most eye specialists, did not accurately identify problems in visual acuity.

The inaccuracy of the FPT was apparent, when the researchers compared test results between FPT and ATS among children who were able to take both tests.

Dr. Sandra Block, a professor of optometry at the Illinois College of Optometry in Chicago, isn't surprised that the FPT isn't foolproof.

"What we want to find out in children that are preschool age is a decrease in vision due to amblyopia such as nearsightedness, farsightedness or a significant astigmatism, which leads to 'lazy eye,'" Block said. "One test that looks at visual acuity in the good seeing eye does not give a full picture. We may be missing some of the amblyopia."

"It's very important in screening that we look at each eye individually, otherwise, we are going to miss some of those kids," Block said.

In another report in the same journal, the same research team noted a slight difference between visual acuity in black and white children among the same children tested in the first study.

"About 1 percent of preschool children who could have vision tested were found to have decreased vision," Friedman said. "Half of these could benefit from wearing glasses."

The researchers found that the prevalence of decreased visual acuity was relatively low, affecting 1.2 percent of white children and 1.8 percent of black children. The difference between the two groups was not statistically significant, the researchers noted.

The most common causes of decreased visual acuity were nearsightedness, farsightedness and/or astigmatism. Only one child was legally blind.

Unidentified vision loss in preschool children is uncommon, Friedman said. "Increased efforts to identify children with vision loss at such a young age are probably not necessary," he said.

Block agrees vision problems among preschoolers are relatively rare, but parents should still have their children's eyes tested.

"The chances of vision problems in the preschool population are low, but they do exist," Block said. "The screenings are an important piece of the child's early evaluation and are usually done by a pediatrician as part of routine health care."

If a child appears to have a vision problem, he or she should be seen by a specialist, Block said. "Vision is an issue, and the best time to address it is early on, before it develops into true problems that cannot be corrected."

SOURCES: David S. Friedman, M.D., Ph.D., associate professor, ophthalmology and international health, Wilmer Eye Institute, Johns Hopkins School of Medicine, Baltimore; Sandra Block, O.D., professor, optometry, Illinois College of Optometry, Chicago; October 2008, Ophthalmology